Comptes Rendus
The new International System of Units / Le nouveau Système international d'unités
Silicon spheres for the future realization of the kilogram and the mole
Comptes Rendus. Physique, Volume 20 (2019) no. 1-2, pp. 64-76.

New definitions of the units for amount of substance – the mole – and for mass – the kilogram – will presumably come into force on the World Metrology Day 2019. In the revised SI, the mole will be defined by a fixed value of the Avogadro constant and the kilogram by a fixed value of the Planck constant. The X-ray-crystal-density (XRCD) method has been used for the determination of these fundamental constants by counting the number of atoms in 28Si-enriched spheres. Thus, silicon spheres will – after the redefinition – be used to realize the definitions of the mole and the kilogram. This is possible by SI-traceable measurements of lattice parameter, isotopic composition, volume, and surface properties, yielding a relative standard uncertainty below 2⋅10−8. Whereas this high accuracy is only reached with isotopically enriched silicon, it is also planned to use natural silicon spheres on a slightly lower level of accuracy. The future definitions will allow also new realization methods using silicon, in particular for small mass values.

De nouvelles définitions des unités de quantité de matière (la mole) et de masse (le kilogramme) entreront vraisemblablement en vigueur lors de la Journée mondiale de la métrologie de 2019. Dans le Système international révisé, la mole sera définie par une valeur fixe de la constante d'Avogadro et le kilogramme par une valeur fixe de la constante de Planck. La méthode de mesure de densité par cristallographie aux rayons X (XRCD – X-ray crystal density) a été utilisée pour déterminer ces constantes fondamentales par comptage du nombre d'atomes dans des sphères de silicium enrichi en silicium 28. Après la redéfinition, les définitions de la mole et du kilogramme seront ainsi réalisées à l'aide de sphères de silicium. Ceci est possible en effectuant des mesures des paramètres de réseau, de la composition isotopique, du volume et des propriétés de surface traçables au SI. On obtient alors une incertitude-type relative inférieure à 2108. Alors qu'une telle précision ne peut être atteinte qu'avec du silicium enrichi isotopiquement, il est également prévu d'utiliser des sphères de silicium naturel avec un niveau de précision légèrement inférieur. Les futures définitions permettront également de mettre en oeuvre de nouvelles méthodes de réalisation faisant appel au silicium, en particulier pour de faibles valeurs de masses.

Published online:
DOI: 10.1016/j.crhy.2018.12.005
Keywords: XRCD method, Silicon, Spheres, Revised SI, Planck constant, Avogadro constant
Mot clés : Méthode XRCD, Silicium, Sphères, SI révisé, Constante de Planck, Constante d'Avogadro

Horst Bettin 1; Kenichi Fujii 2; Arnold Nicolaus 1

1 Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
2 National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan
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Horst Bettin; Kenichi Fujii; Arnold Nicolaus. Silicon spheres for the future realization of the kilogram and the mole. Comptes Rendus. Physique, Volume 20 (2019) no. 1-2, pp. 64-76. doi : 10.1016/j.crhy.2018.12.005. https://comptes-rendus.academie-sciences.fr/physique/articles/10.1016/j.crhy.2018.12.005/

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